The present invention relates to a cathode ray tube having an electron gun employing an indirectly heated cathode, and particularly to a cathode ray tube having a heater for the indirectly heated cathode with ease of its welding and its reliability improved.
Cathode ray tubes such as TV picture tubes and display tubes are widely used as a display means in various kinds of information processing equipment because of their capability of high-resolution image reproduction.
The cathode ray tubes of this kind include an evacuated envelope comprising a panel portion having a phosphor screen formed of phosphors coated on its inner surface, a tubular neck portion and a funnel portion for connecting the panel portion and the neck portion, an electron gun housed in the tubular neck portion comprising an electron beam generating section including an indirectly heated cathode, a control electrode and an accelerating electrode, and a main lens section for focusing an electron beam generated in the electron beam generating section onto the phosphor screen, and a deflection yoke mounted around the funnel portion for scanning the phosphor screen with the electron beam from the electron gun.
FIG. 5 is a schematic cross-sectional view of a shadow mask type color cathode ray tube for explaining an example of a structure of a cathode ray tube. Reference numeral 1 denotes a panel portion, 2 is a funnel portion, 3 is a neck portion, 4 is a phosphor screen formed of phosphors coated on the inner surface of the panel portion, 5 is a shadow mask serving as a color selection electrode, 6 is a magnetic shield for shielding an external magnetic field (the Earth""s magnetic field) for preventing the Earth""s magnetic field from having adverse influences on the trajectory of electron beams. Reference numeral 7 denotes a deflection yoke, 8 is external magnets for beam adjustment, 9 is an electron gun provided with indirectly-heated cathodes for emitting three electron beams and 10 are the three electron beams only one of which is shown.
The three electron beams 10 from the electron gun 9 are modulated by video signals from an external signal processing circuit (not shown), respectively, and are projected toward the phosphor screen 4. The electron beams 10 scan the phosphor screen 4 two-dimensionally by being subjected to the horizontal and vertical deflection magnetic fields generated by the deflection yoke 7 mounted around the transition region between the neck portion 3 and the funnel portion 2. The shadow mask 5 reproduces a desired image by passing the three electron beams through a large number of apertures therein to the phosphor screen such that each beam impinges upon and excites only one of the three kinds of color phosphor elements in the phosphor screen.
FIG. 6 is a side elevation view of an electron gun for explaining an example of a structure of an electron gun used for the color cathode ray tube shown in FIG. 5. The electron gun comprises a control electrode (the first grid electrode or G1) 11, an accelerating electrode (the second grid electrode or G2) 12, focus electrodes (the third grid electrode or G3, the fourth grid electrode or G4, and the fifth grid electrode or G5) 13, 14, 15, an anode (the sixth grid electrode or G6) 16, and a shield cup 17 physically retained in axial predetermined spaced relationship in the order named by multiform glass 20, and the respective electrodes are electrically connected to respective stem pins 18a implanted in a stem 18 by welding a tab or a lead provided to the electrodes, to the stem pins 18a. 
In this electron gun, an indirectly heated cathode structure 21 is spaced closely from the electron beam apertures in the control electrode 11 toward the stem 18, and has heaters for heating the electron-emissive surfaces.
Reference numeral 19 denote bulb spacer contacts for centering the central longitudinal axis of the electron gun coincident with the axis of the neck portion by pressing resiliently against the inner wall of the neck portion and for effecting delivery of an anode voltage from the internal conductive coating coated on the inner walls of the funnel and neck portions to the electron gun.
The indirectly heated cathodes 21, the control grid 11 and the accelerating electrode 12 form an electron beam generating section (a triode portion). The focus electrodes 13 to 15 accelerate and focus the electron beams emitted from the electron beam generating section, and then a main lens formed between the focus electrode 15 and the anode 16 focuses the electron beams onto the phosphor screen.
The stem 18 is fused to close the open end of the neck portion 3 of the vacuum envelope, and signals and voltages from external circuits are applied to the respective electrodes via the stem pins 18. The external magnets 8 (the magnet assembly) for beam adjustment shown in FIG. 1 correct errors in landing of the electron beams on the phosphor elements caused by a misalignment in axis or a rotational error between the electron gun and the panel portion, the funnel portion and the shadow mask.
FIG. 7 is a cross-sectional view of the indirectly heated cathode structure 21 shown in FIG. 6. The indirectly heated cathode structure 21 comprises bead supports 22, an eyelet 23, heater supports 24, a heater 25, a base metal 27 for supporting an electron-emissive material 26, a cathode support sleeve 28 and a cathode cylinder 29.
The indirectly heated cathode structure 21 is fixed on multiform glass 20 by the eyelet 23 and the bead supports 22. The heater 25 housed within the cathode support sleeve 28 are fixed by welding its ends to the heater support 24.
FIGS. 8A and 8B are illustrations of a structure of the heater, FIG. 8A being a side view of the heater and FIG. 8B being an enlarged fragmentary cross-sectional view of the encircled portion designated xe2x80x9cAxe2x80x9d in FIG. 8A. As shown in FIG. 8B, the heater 25 comprises a tungsten wire 31 spirally wound, an alumina insulating layer 32 coated around the tungsten wire 31, and a blackened fine-powder tungsten layer 33 coated around the alumina insulating layer 32. The blackened layer 33 is intended for lowering the temperature required of the heater 25 by improving the heat radiation from the heater 25, and consequently improving the reliability of the heater.
In FIG. 8A, reference character HL denote a leg portion of the heater 25 comprised of tungsten wires spirally wound in three layers, HM is a major heating portion of the heater 25 formed by winding spirally in a large diameter a tungsten coiled wire having been wound initially spirally in a small diameter (hereinafter referred to merely as a coiled coil portion), HA is a portion coated with alumina, HB is a blackened portion covered with the blackened fine-powder tungsten layer 33, HE is a portion not covered with alumina and reference numeral 39 denotes a hollow formed after dissolving and removing a molybdenum mandrel.
A method of forming the leg portion HL of the heater 25 in the three layers of tungsten wires is disclosed in Japanese Utility Model Publication No. Sho 57-34671 (Japanese utility model application No. Sho 51-167255, laid-open date: Jul. 12, 1978, Publication date: Jul. 30, 1982).
FIGS. 9A-9E illustrate sequence of steps in a conventional method of fabricating the conventional heater.
In FIG. 9A, a tungsten wire 31 is wound spirally forward as indicated by an arrow P around a molybdenum mandrel wire 40 up to point A.
Next, as illustrated in FIG. 9B, the tungsten wire 31 is wound spirally backward from point A to point B as indicated by an arrow Q.
Then, as illustrated in FIG. 9C, the tungsten wire 31 is wound spirally forward again from point B to point C over a centerline CL for bending in a subsequent process as indicated by an arrow R, forming a three-layer winding portion TWA ranging from point A to point B.
Next, as illustrated in FIG. 9D, the tungsten wire 31 is wound spirally backward from point C to point D as indicated by an arrow S.
Next, as illustrated in FIG. 9E, the tungsten wire 31 is wound spirally forward from point D to point E as indicated by an arrow T, forming a three-layer winding portion TWB ranging from point C to point D.
The tungsten wire thus wound around the molybdenum mandrel wire 40 is cut at the respective centers F, G of the three-layer winding portions TWA and TWB to provide a tungsten wire winding having a length HQL for one heater with the leg portions THLA, THLB of three-layer winding and is formed into a final shape by folding at the centerline CL as shown in FIG. 8A. Then, the molybdenum mandrel wire 40 is dissolved with acid, leaving a hollow 39 as shown in FIG. 8B.
The heater having the leg portions of the above three-layer winding structure provides the following advantages:
(i) prevention of breaks of a tungsten wire by sparks within a cathode ray tube,
(ii) reduction of power consumption by concentration of heat generation in a coiled coil portion 35 due to low resistance of the three-layer winding portions,
(iii) improvement in workability in the operation of welding a heater,
(iv) suppression of heat generation in the portion not covered with alumina caused by an overcurrent upon power turn on.
The tungsten wire for heaters are very thin, and are usually 30 xcexcm to 50 xcexcm in diameter. The structure of the wound thin wires is very weak in mechanical strength, and welding of heaters to a heater support requires a great deal of skill. The three-layer winding structure improves workability in welding heaters, and suppresses occurrences of breaks of heaters by sparks or overcurrents upon power turn on.
Recently it has been difficult to perform operations requiring skill such as welding of heaters. Although the above prior art has improved the workability in welding of heaters, sufficient consideration has never been given to a following problem in heater welding by unskilled workers or by machines, that is, the mechanical strength of the leg portions wound in three layers of heaters is not sufficient for the operation of manually inserting a heater into a cathode support sleeve or for the automatic operation of detecting weld positions of a heater and then welding the heater.
Cracks sometimes occur in the alumina-coated portion in the vicinity of weld points in the operation of welding the portion not covered with alumina, of the leg portions wound in three layers to a heater support. For prevention of the cracks, rigidity of the three-layer winding portion needs to be reduced by winding the tungsten wires at a coarser pitch in that portion, but a problem arises that the workability in welding deteriorates.
It is an object of the present invention to provide a cathode ray tube having an electron gun employing an indirectly heated cathode structure free from cracks in the alumina insulating layer of leg portions of the heater without deterioration in welding workability by solving the above problems with the prior art.
To accomplish the above object, according to a preferred embodiment of the present invention, there is provided a cathode ray tube comprising an evacuated envelope comprising a panel portion, a neck portion, a funnel portion for connecting the panel portion and the neck portion and a stem having a plurality of pins therethrough and being sealed to close the neck portion at one end thereof, a phosphor screen formed on an inner surface of the panel portion, an electron gun housed in the neck portion, the electron gun comprising an electron beam generating section comprising an indirectly heated cathode structure, a control electrode and an accelerating electrode, and a main lens for focusing an electron beam from the electron beam generating section onto the phosphor screen, and a deflection yoke mounted around a vicinity of a transitional region between the neck portion and the funnel portion for scanning the electron beam on the phosphor screen, the indirectly heated cathode structure comprising a metal sleeve, a base metal having an electron emissive material coating on an outer top surface thereof and attached to one end of the metal sleeve, and a heater positioned within the metal sleeve, wherein the heater comprises a major heating portion having a spirally wound heating wire and leg portions disposed at ends of the major heating portion, each of the leg portions comprises a first multilayer winding portion having heating wires wound spirally in a plurality of layers and a second multilayer winding portion disposed intermediate between the major heating portion and the first multilayer winding portion and having heating wires wound in a plurality of layers, the major heating portion and at least a portion of the second multilayer winding portion are covered with an insulating coating, the heater is welded to electrical conductors for applying a voltage thereto at the first multilayer winding portion, and layers in the second multilayer winding portion is at least three in number and layers of the first multilayer winding portion is larger in number than layers in the second multilayer winding portion.
The present invention is not limited to the above structure, and various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.